UL VM38 tuning info

[Eb74UL]

Would anybody have any advice on a VM38 setup on a UL?

I have not fired up the motor yet but I have a VM38 on the engine and it is setup just like for a shovel. Would the UL like it richer or leaner or just the same? My engine has iron heads so I would think it will run hotter than an alum head engine and may like it a bit richer than a shovel setup? I am way up north so no very hot temperature here.

[kitabel]

setup just like for a shovel

What jetting parts?

[Eb74UL]

Main: 330
Pilot: 30
Needle: Jet 166-Q2
Needle: 6DP1
Air Jet: 2.0
Slide: 2.5

[38ulh]

After messing a little with my jets with advice off of this forum my set up is exactly what you have now with 3/4 turn out on air. My bike starts Idles and runs great. I could probably go one click leaner across the board but with a UL I like to error to the side of rich. P.S. needle clip in the middle.

[Eb74UL]

Thanks to you Kitabel and 38ulh. Happy to know I should be OK. Gas tank clearance makes it a little harder than usual to remove the top of the VM38 and so I wished to be as close to running right as possible. Hopefully I don't have to mess with it too much. And yeah, clip in the middle...

[Eb74UL]

Well, just got the bike started, I started to play with the idle screw and as I screwed it in and reduced the idle speed, I ended up screwing it all the way in and the bike still likes it... So what do you guys think? go down to a 1.5 air jet?

[38ulh]

I'm running the same set up as you. When I was tuning it I had very little adjustment at the idle air. (1/4 turn out) I went down a jet size from 35 to 30 and it ran best at 3/4 turn out. All my bikes have room for tuning improvements, But I'm no master tuner. I got all my UL knowledge off of this board. I think your on the right track. Hopefully one our local tuners will chime in.

[Eb74UL]

Yeah, I should have written "idle air screw" instead of just "idle screw" in my post, but you understood.

For clarity for all, by screwing my idle air screw in, I am going richer in my idle mixture and engine speed was increasing, meaning it required the richer mixture. Then I unscrewed the round slide idle screw until my engine RPW went back down and repeated the process. Bike like it all the way to fully closed. Pretty sure the carb slide is not so loose as to let air through.

You just reminded me that I could try a different idle fuel jet as well. However, one thing that puzzles me on your post is that you went leaner in the idle fuel jet (from 35 to 30) and also leaner by opening the air screw further out (from 1/4 to 3/4). I would have though you would have needed to go richer on the fuel jet and leaner on the air screw.

Eb

[38ulh]

It doesn't sound like our carbs are going to work with the exact same set up. I started with a used carb when I was done fooling with it I think I ended up with what you have now. I know I changed both jets. In the beginning I was idling too rich. All I had to work with was 1/4 turn (engine would die). My research and reading told me once I had my jets close I should be 1 1/2-2 turns out. I have a procedure in my notes (off the internet where everything is gospel). 1. Get engine started and warm. 2. use idle speed to set engine at high idle (my ears). 3. adjust IA in or out till you get the fastest idle. 4. now use idle speed to lower to you preferred idle. 5. Now very fine tune with IA till your happy wit throttle response. IA screw in equal richer out equal leaner. P.S. I've always considered myself one of the worst tune up men in the world so FWIW. But keep us posted I love flatheads.

[Eb74UL]

38ulh,
You got the procedure figured out, get the Mikuni book if you do not have it, it is really good at explaining the tuning procedure. The tuning section shows the impact of the different carburetor components on mixture at different throttle openings. I've made a tape of throttle openings on my handlebar as they suggest in the book to help understanding which component to change. An easy example is mainjet, it has an impact at wide open throttle, so you don't change that component if you have a mixture issue at 1/4 throttle opening... This is not rocket science just takes patience... which I am unfortunately getting short of...

[Eb74UL]

To those interested, This is where I end up:
Main: 330 (same)
Pilot: 35 (instead of 30)
Needle: Jet 166-Q5 (instead of 166-Q2)
Needle: 6DP1 (same) middle groove
Air Jet: 2.0 (same)
Slide: 2.5 (same)

The bike needed a lot of choke to start and would not respond when opening the throttle, the idle air screw had to turned almost all the way in for the idle to settle. On the road at 55 mph, the bike felt underpowered and plugs where quite white.

1st change: Changed pilot jet from 30 to 35
Bike needed less choke to start and responds to throttle opening, air screw now set at 1 1/2 turn.
Still feels unresponsive at 55 mph and plugs still white.

2nd change: Changed needle jet from 166-Q2 to 166-Q4
Bike improves in throttle response at 55 mph but plugs still white

3rd change: Changed needle jet from 166-Q4 to 166-Q6
Bike does improve in throttle response at 55 mph but a bit sluggish at steady lower speeds, plugs still white but shoulder has darkened.
However, bikes puts out loud pops in the exhaust during deceleration, I suspect unburned fuel in the exhaust and likely too rich now.

4th change: Changed needle jet from 166-Q6 to 166-Q5
Bike still pulls good at 55 mph, less sluggish at lower speeds and pops in the exhaust are gone.

At 55 mph my throttle opening is about at 1/4. At that opening, the Mikuni books lists 2 components to change, the throttle valve (the piston) and the needle jet (the tube). My experience with going from 2.5 to 2.0 on the throttle valve is that it enriches the mixture quite a lot and so I did not want to go that way. I feel I would have likely have to go back down on the idle jet to 30 and would likely have the same air screw issue.

Bike runs good now, runs cooler, no pops in the exhaust... So after taking the carb off and on 4 times, I will run it this way for a while and see. Plug are new so coloring may not be very accurate right now.

Eb